1. Field of the Invention
The present invention relates to an internal grinding method for grinding an inner surface of a work having a straight-line generatrix shape while oscillating a grindstone relative to the work, and an internal grinding machine useful in carrying out this method.
2. Description of the Related Art
Internal grinding of a cylindrical small part such as a rolling bearing inner ring is performed by moving (oscillating) a grindstone forward and backward within a work along an axis of a X-axis slide table while carrying out intermittent in-process measurement. When an inner surface of a small work 2 is ground, a dimensional restriction does not allow a grindstone 1 attached at an end of a wheel spindle 4 and an in-process gauge 3 to keep simultaneous contact with a ground surface of the work 2. As shown in FIGS. 6 and 7, the grindstone 1 and the in-process gauge 3 are therefore oscillated synchronizedly so as to perform size measurement on the work 2 by inserting the in-process gauge 3 into the work 2 when the grindstone 1 moves backward.
By this method, conventionally, grinding is generally finished when intermittently measured value reaches over a predetermined target-size value. In this case, however, the grinding cannot be always finished at a real target size. Accordingly, the dimensional accuracy is restricted. With regard to this aspect, in order to improve the dimensional accuracy, there have been proposed a method and a device in which predictive control is performed. On the basis of the intermittently measured in-process signal, the measuring device is retreated and the feeding of a grindstone is stopped on the basis of the predictive control signal (Japanese Patent Examined Publication No. Sho. 53-14797). In this method, the measuring device retreats just before the target size and terminates its size measuring operation. The grindstone is retreated from the work when the predicted size gets the target-size.
In the above-mentioned method and device, grinding is finished (the grindstone is retreated, from the work) at an arbitrary position in an oscillation stroke in accordance with the predictive target size signal. As a result, the contact position between the work and the grindstone is not fixed when grinding is terminated, which causes difference in diameter in the inner surface of the work. For such a reason, there is a disadvantage that the generatrix shape of the work deteriorates.
In order to solve the foregoing problem, there has been proposed a method in which an predictive operation for the work size is carried out with intermittently measured data by an intermittent in-process measuring device, and the oscillation speed or the remaining number of oscillation after the predictive operation is controlled so that a grindstone comes to a forward end of its oscillation just when the work size comes into a certain range of its target size (Japanese Patent Unexamined Publication No. Hei. 4-310368). In this method, a grindstone always stops grinding in full contact in axial direction, so that not only the dimensional accuracy of the work but also the generatrix shape accuracy and the cylindrical accuracy are improved.
In actual grinding work, the wheel spindle 4 is bent by normal grinding force as shown in FIGS. 8 and 9. As a result, when the oscillation of the grindstone 1 is backward, the forward end of the grindstone 1 grinds the surface of the work 2 locally. Specifically, in the above-mentioned grinding method accompanied with intermittent measurement, grinding width Wb becomes about xc2xd of whole length Wf of the work when the grindstone moves backward, as shown in FIG. 8. In the conventional method, however, the wheel spindle 4 is oscillated in parallel with the moving direction of the X-axis slide table which performs trueing of the grindstone, so that the normal grinding force is not reduced in proportion to the reduction of the grinding width (Wfxe2x86x92Wb) when the grindstone moves backward. As a result, the normal grinding force per unit grinding width becomes excessive so that the grindstone 1 machines the work 2 at its forward edge 1a sharply. When the generatrix shape of the work deteriorates on a large scale in the process of grinding, the deterioration affects the dimensional accuracy of the work when the grinding is terminated. It is therefore necessary to keep the generatrix shape of the work properly during machining.
The above-mentioned conventional methods are not intended to improve the work shape on the way of grinding, but to correct the work generatrix shape, which has deteriorated in the machining cycle, just before finishing gringing, without lowering the dimensional accuracy. For example, in the foregoing grinding control method in Japanese Patent Unexamined Publication No. Hei. 4-310368, the grindstone always stop grinding at a forward end of an oscillation stroke, but deterioration of the generatrix shape of a work caused by sharp grinding at the forward end of the grindstone during its oscillating operation cannot be removed completely when grinding is terminated. In addition to the deterioration of the work shape, in the conventional methods, there is a problem of local abrasion of the grindstone due to large grinding resistance produced locally when the grindstone moves backward, so that the life of the grindstone is shortened.
Therefore, the applicant of the present invention has therefore disclosed a grindstone oscillation method in which a grindstone was oscillated in a direction inclined relative to the forward/backward moving direction of a X-axis slide table which performs trueing of the grindstone. In order to realize this method, the applicant developed an internal grinding machine shown in FIG. 10 (Japanese Patent Unexamined Publication No. Hei. 5-285808). This configuration will be described with reference to FIGS. 10 and 11.
An X-axis slide table 7 and a work head-stock (not shown) which holds a work 2 in opposition to the moving direction of the table 7 are mounted on a bed of a grinding machine. Further, an in-process measuring device 9 having an in-process gauge 3 is provided in the vicinity of the head-stock. An oscillation unit 6 is mounted on the X-axis slide table 7 through a vertical pivot shaft 115 which can be inclined relative to the table 7. In addition, a table 116 which reciprocates along a guide portion 6a is mounted on the oscillation unit 6. A wheel spindle device 5 which can be inclined relative to the table 116 is mounted on the table 116. In the illustrated example, the wheel spindle device 5 which can be moved around a pivot 114 (see FIG. 10) provided on the working table 116, which is independent of the motion of the oscillation unit 6 relative to the X-axis slide table 7. In this connection, this pivotal structure may be replaced so that arcuate irregular engagement portions each having an appropriate radius around the pivot shaft 115 of the oscillation unit 6 is formed on the upper surface of the table 116 and the lower surface of the wheel spindle device 5 respectively, and the wheel spindle device 5 can be tilted relative to the working table 116 along these arcuate engagement portions. Four piezoelectric force transducer 117 for detecting normal grinding force are mounted between the wheel spindle device 5 and the table 116. The normal grinding force applied to the grindstone 1 in grinding operation is sum of the values detected by the four force transducers 117.
An arcuate groove 118 with the pivot shaft 115 as a center thereof is formed in the upper surface of a rear end of the oscillation unit 6. Through this arcuate groove 118, a clamp bolt 119 for fixing the oscillation unit is screwed into the X-axis slide table 7 from the upper surface of the oscillation unit 6. A hydraulic cylinder (not shown) is mounted on the X-axis slide table 7. A piston rod 120 of the hydraulic cylinder is engaged with a head portion of the clamp bolt 119. The clamp bolt 119 is rotated by the forward/backward movement of the piston rod 120 so that the oscillation unit 6 is fastened or unfastened to the X-axis slide table 7 by the bolt head portion to be thereby automatically clamped or released from clamping. Though not illustrated, the wheel spindle device 5 can be also tilted and then clamped onto the working table 116 by a suitable clamping device.
The oscillation unit 6 has a motor, a linear guide and an eccentric cam. By driving the motor, through the eccentric-cam and the linear guide, the oscillation unit 6 gives table 116 located above sine-wave reciprocating motion. (oscillating operation) along the guide 6a. The oscillation unit 6 is inclined at an angle xcex8 with respect to the moving direction of the X-axis slide table 7. This angle is regarded as an oscillation angle. When an initial oscillation angle xcex8 is decided, grinding is firstly performed with the oscillation angle xcex8=0. The normal grinding force is detected by the force transducers 117, and the angle xcex8 is calculated in an arithmetic circuit 121 (See the expression (14) in the embodiment according to the present invention.) Then, the clamping device for the oscillation unit 6 is released, and the oscillation unit 6 is rotated by the angle xcex8 around the pivot shaft 115 by a linear motor 113 through a motor driving device 122 on the basis of an output signal from the arithmetic circuit 121. Next, the oscillation unit 6 is fixed onto the X-axis slide table 7 by the foregoing clamping device for the grindstone oscillation unit. At the same time, the wheel spindle device 5 is rotated reversely by the same angle xcex8 relative to the working table 116 so as to be clamped. As a result, a grindstone generatrix is kept parallel to the moving direction of the X-axis slide table 7. Then, the X-axis slide table 7 is moved forward so that the grindstone 1 at the left side of the wheel spindle device 5 is inserted into the work 2. A motor (not shown) on the bed gives feed motion to start oscillation grinding of the work 2. The direction of the oscillation is inclined at the angle xcex8 with respect to the grindstone generatrix without load.
Trueing of the grindstone 1 is performed in the forward/backward motion of the X-axis slide table 7 by a truer 8 provided in parallel with the moving direction of the X-axis slide table 7.
In the above-mentioned grindstone oscillation method in internal grinding and the internal grinding machine therefor disclosed in Japanese Patent Unexamined Publication No. Hei. 5-285808, a grindstone is moved backward at the oscillation angle xcex8 inclined relative to the X-axis in a direction that the grindstone moves away from the work. As a result, the bending quantity of the wheel spindle is reduced when the grindstone moves backward, and the normal grinding force per unit grinding width is lowered. Accordingly, if the above-mentioned oscillation angle is selected suitably, the normal grinding force per unit grinding width can be made equal between two cases when the grindstone moves most backward and forward. Thus, sharp grinding is prevented in the position where the grindstone moves backward so that internal grinding without any difference in level on the ground surface of the work can be attained as shown in FIG. 12A in comparison with the internal grinding shown in FIG. 12B to which this method is not applied. The method is extremely useful in this term. However, the method provides a configuration in which the wheel spindle is oscillated in a direction inclined relative to the moving direction (axial direction of the grindstone) of the X-axis slide table which performs trueing of the grindstone. As a result, an oscillation unit 6 having a table 116 supported on the X-axis slide table 7 so as to perform an oscillating operation is required in addition to the X-axis slide table 7. Thus, there has been a problem that mechanisms are increased in number and in complexity so that the grinding machine as a whole becomes expensive.
The object of the present invention is to provide an internal grinding method and an internal grinding machine which does not need to provide the oscillation unit, and the slide table on it and so on in the invention (Japanese Patent Unexamined Publication No. Hei. 5-285808); in which, with a simple structure with low cost, normal grinding force per unit grinding width can be prevented from being excessive when a grindstone moves backward; and in which the shape of a work can be improved and the life of the grindstone can be long.
The present invention provides an internal grinding method for grinding an inner surface of a work having a straight line generatrix shape while performing an in-process inner-diameter size measurement intermittently through high-speed oscillation synchronized between a grindstone and an in-process gauge; wherein a circumferential surface of the grindstone is trued into a shape inclined with respect to a moving direction of a X-axis slide table moving forward/backward relative to the work so that normal grinding force when the X-axis slide table moves backward is reduced, while the work is supported with inclination with respect to the moving direction of the X-axis slide table so that the X-axis slide table is oscillated in the moving direction. Further, the present invention provides an internal grinding machine comprising: an X-axis slide table for performing an oscillating operation along an axis of a grindstone; a wheel spindle device having the grindstone at its end, the grindstone having a circumferential surface with a tapered shape; a cross slide table for moving in a direction perpendicular to the direction of the X-axis; a grindstone trueing device held on the cross slide table and for traverse trueing the circumferential surface of the grindstone; a work supporting device held on the cross slide table and for supporting a work inclined relative to the X-axis so that an inner surface of the work contacts with the circumferential surface of the grindstone; an in-process size measuring device held on the work supporting device inclined together with the work; and a control device for controlling forward/backward motion of the X-axis slide table and feed motion of the cross slide table so that the circumferential surface of the grindstone is trued with tapered shape.
Trueing of a grindstone is performed by relative movement between a X-axis slide table and a truer. According to the present invention, trueing is carried out as the surface of the grindstone is inclined in a tapered state relative to the oscillation direction based on the X-axis slide table. Thus, the grindstone moves backward in a direction to separate from a work when the grindstone moves backward. In addition, the work, if it needs cylindricity, is supported in advance so as to be inclined in a direction to provide proper cylindricity when the bending of the X-axis at the time of grinding is applied in the direction of the grindstone surface trued in inclination. As a result, the bending quantity of a wheel spindle is reduced when the grindstone moves backward, so that normal grinding force per unit grinding width is lowered. Thus, if the trueing angle of the grindstone is selected suitably, the normal grinding force per unit grinding width when the grindstone moves most backward becomes smaller than that in the related art.